In ocean floor surveying and subbottom mapping with sonar for different commercial and noncommercial applications, it would be desirable to form and aim a narrow sonar beam from a submerged vessel having both a transmitter and receiver towed at some speed, such as 7 knots. It has been discovered that the Doppler effect produced in a frequency chirp may be used to advantage to aim a beam, and that control of the time-bandwidth product of the chirp can be used to form a narrow beam thereby to increase the effectiveness of sonar mapping and surveying.
Mapping is, of course, only one application of a sonar system embodying the invention. It may also be used in other applications, such as in selective target identification. For example, assume a command ship wishes to be able to determine the presence of a particular submarine in an area having many other targets in the water at about the same range. The echo returns from all the targets may make it very difficult to determine the presence of the submarine if all targets have about the same relative velocity as the submarine. If the particular submarine returns a transponder produced echo that is Doppler shifted more than expected echo returns from any other target in the water, the submarine can be easily identified by correlating the signal received from the transponder with its replica at the receiver. Still other applications will occur to those skilled in the art.
Although the invention will be first described in terms of synthesizing and aiming the narrow beam in real time, it can be readily appreciated that in practice the transmitted and received sonar signal may be recorded, such as on magnetic tape, for later processing. In the later processing, the technique to be described can be applied as though the data were being received in real time. An advantage is that the same data can be processed repeatedly, each time effectively aiming the beam in the direction of a different target. It should also be understood that the basic concept of the invention may be used with radar, since electromagnetic wave energy will respond to the same principles in respect to the Doppler effect occurring when the source and the receiver are in motion relative to each other, as with acoustical wave energy.
It will be shown that the ratio of the velocity of the relative motion to the speed of sound in water is an important factor in synthesizing a narrow sonar beam. The corresponding ratio of the relative velocity to the speed of light is likewise an important factor in synthesizing a narrow radar beam. As will be explained more fully hereinafter, the half angle of the beam decreases as that ratio increases, but since there is a practical limit in producing relative velocity that is commensurate to the speed of light, it is not generally practical to try to reduce the width of a radar beam for earth bound applications. However, spaceflight and satellite applications may produce a relative velocity that is commensurate to the speed of light to permit the present invention to be utilized. One application may be for a spaceship (or earth station) to track the position of another spaceship or satellite.